The invention relates to an aircraft comprising a symmetry plane, two wings, a fuselage, a horizontal tail plane (HTP) and with means for a premature breakdown of a wing vortex pair behind the aircraft in flight, which pair is generated in the tip area of the wings, has a first spin direction and descends in the near field behind the aircraft.
The invention deals in particular with the premature breakdown of the wing vortex pair behind an aircraft in landing flight. However, it is also advisable to apply the invention to an aircraft in climbing or cruise flight.
The wing vortices of the wing vortex pair are generated at an aircraft in flight by the rolling up of the vortex sheet shed from the wing tip area. The trailing vortices are particularly strong in landing approach. Especially with the advent of wide body aircraft the wing vortices, due to the trailing vortices they generate, are a hazard for other aircraft since the force of the wing vortices, which hereafter is called circulation, is essentially proportional to the weight of the respective aircraft. Hence, particularly heavy aircraft generate particularly strong wing vortices. Thus, the horizontal spacings between landing aircraft, that are necessary for air traffic safety, increase with the weight of the aircraft in front and are several nautical miles with the presently common wide body aircraft, such as Boeing type B-747. It is these large horizontal spacings for the avoidance of wake vortex encounters that prevent a marked increase in the passenger volumes per hour by even larger capacity aircraft since the admissible landing frequency of such larger aircraft further decreases with their increasing weight.
A method for a premature breakdown of wing vortex pairs is known from Ludwieg: xe2x80x9cVortex breakdownxe2x80x9d, DLR-Report 70-40, 1970. The paper shows, that a vortex breaks down due to the Rayleigh-Ludwieg instability, if another vortex with an opposite rotation appears on its periphery. The condition that it appears on the periphery has been formulated by Steven C. Rennich et al.: xe2x80x9cA Method for Accelerating the Destruction of Aircraft Wake Vorticesxe2x80x9d, AIAA 98-0667, 1998. This publication deals with theoretical models and calculations that are restricted to an external vortex pair as a wing vortex pair and an internal vortex pair with opposite spin direction as a interfering vortex pair. The basic assumption is that, under practical conditions, the external vortex pair is generated at the wing tips and the outside edges of the trailing and leading edge flaps while the internal vortex pair, due to the lift loss, occurs over the fuselage and at the inside edges of the landing flaps. Though a general observation is made that under practical conditions the entire vortex sheet should be considered, there is no reference to any specific further vortex sheets and vortex pairs. Regarding the internal vortex pair it is stated that its relative circulation and relative span in relation to the circulation and the span of the wing vortex pair have to be in a certain range so that the structure of the wing vortex pair can be destroyed due to as a result of the interaction with the interfering vortex pair behind the aircraft in flight. However, if the circulation of the interfering vortex pair is too high the interaction between the two single vortices of the interfering vortex pair is such that they escape to the above and thus the desired interaction, i.e. to destroy the structure of the wing vortices descending behind the aircraft, will not take place. No concrete actions are outlined to prevent the circulation of the interfering vortex pair from becoming too high. In addition, the consideration of the vortex sheets that are shed from the inside span area of an aircraft by only one interfering vortex pair is a very crude simplification since, for instance, vortex sheets that are shed from the aircraft near the horizontal tail plane have not yet rolled up into a defined vortex pair. A less crude simulation of the shed vortex sheets shows that the latter do not escape completely to the above as one interfering vortex but that parts of such already descend with the wing vortices while, according to Rennich et al., their total circulation should make them wholly escape above. Hence, the criterion proposed by Rennich and Lele is replaced by a criterion that is further described below and which considers the vortex sheets more profoundly than the criterion adopted by Rennich and Lele.
This invention is aimed at making use of the interfering vortex pair that is already existing at an aircraft in flight for a premature breakdown of the wing vortex pair to basically reduce its circulation.
Briefly described, this object is achieved by an aircraft comprising a symmetry plane, two wings, a fuselage, a horizontal tail plane and means for a premature breakdown of a wing vortex pair +/xe2x88x92xcex93M, which is generated in the wing tip area at an aircraft in flight, which has a first spin direction and which descends behind the aircraft, in that the means for said premature breakdown adjust the circulation xcex93P and the relative span bPxe2x80x94 each related to the wing vortex pairxe2x80x94of an interfering vortex pair xe2x88x92/+xcex93P, which is generated between fuselage and wing tips without merging with the wing vortex pair already in the near field behind the wings, which has a second spin direction that is opposed to the first spin direction and which includes portions of both a fuselage vortex pair xe2x88x92/+xcex93F generated in the wing-fuselage junction area, and a tail plane vortex pair xe2x88x92/+xcex93H generated in the wing tip areas (5) of the horizontal tail plane (4), to appropriate values in order to retain the interfering vortex pair xe2x88x92/+xcex93P behind the aircraft within an altitude range of the descending wing vortices of the wing vortex pair.
This invention is based on the finding that the internal vortex pair which absorbs portions of the fuselage vortex and the tail plane vortex pairs, is particularly suited as an interfering vortex pair. The fuselage vortex pair is caused by the lift loss over the fuselage at the wing-fuselage junction area and always has a spin direction opposed to the wing vortex pair. The tail plane vortex pair is generated in the wing tip area of the horizontal tail plane which generates a downward lift in order to stabilize the desired angle of attack of the entire aircraft, so that also the tail plane vortex pair has a spin direction that is opposed to that of the wing vortex pair. Fuselage vortex pair and tail plane vortex pair merge into one internal vortex pair behind the aircraft. On the one hand the span between the two single interfering vortices of this interfering vortex pair is so small and, on the other, the distance to the wing vortex is so large that there is no risk for the merging of the interfering vortex pair with the wing vortex pair already in the near field behind the aircraft. Such a merging with the not yet aged wing vortex pair would cause a certain reduction of its circulation but not the desired premature breakdown of its structure. The reviewed interfering vortex pair with portions of the fuselage vortex and the tail plane vortex pairs typically has such a high circulation in the known wide body transport aircraft that, without a purposeful intervention, it would escape to the above from the area of the descending wing vortex pair due to the small span of its two opposed interfering vortices. As a counteraction the new method features an adjustment of the circulation and the relative span between said interfering vortex pairs to such values that the interfering vortex pair is retained in the altitude range of the descending wing vortices of the wing vortex pair. This means that the circulation of the naturally occurring interfering vortex pair with portions of the fuselage vortex and tail plane vortex pairs usually has to be reduced.
The use of winglets for said reduction can reduce, or, compensate the additional aerodynamic drag of the winglets, which is due to the propulsion they generate by their local oncoming airflow, or even overcompensate it in terms of a net propulsion. The definition for winglets in this context is that these are small wings that are not blown against in the main undisturbed flow direction, i.e. not opposed to the overall moving direction of the aircraft. The air-flow hitting the winglets is part of the flow around the entire aircraft whose direction may considerably deviate locally from the direction of the undisturbed flow, such as at a wing tip.
The interfering vortex pair, which, according to the present invention, is used for a premature breakdown of the wing vortex pair circulation, can also have a portion of a flap vortex pair that is generated in the area of the inside edges of a landing flap pair at the wings. When there is a merging of such flap vortex pair with the fuselage vortex and the tail plane vortex pairs also its portion in the circulation during the retaining of the interfering vortex pairs in the altitude range of the wing vortices is to be considered. It goes without saying that it has to be borne in mind here that flap vortex pairs cannot be observed in sufficient strength in all flight situations and vanish completely when the landing flaps are retracted. However, during the especially interesting landing approach the landing flaps are typically fully taken out. On the other hand, it has to be seen to that the circulation of the interfering vortex pairs is not too grossly reduced by possible winglets when reducing the wing vortices in cruise flight, after the essential portions of the flap vortex and tail plane vortex pairs have fallen away.
The interesting interfering vortex pair can also have a portion of a generator vortex pair that is generated by a pair of vortex generators with the first or second spin direction. The vortex generators can be integrated, for instance, into the structure of the main gear of the aircraft. The arbitrary generation of vortices by means of vortex generators makes it possible to subject both the circulation of an interfering vortex pair and its span to a specific adjustment. The generator vortex pair can have both the second spin direction, which coincides with the interfering vortex pair spin direction, and the first spin direction of the wing vortex pair. In the latter case the generator vortex pair already causes a principal reduction of the interfering vortex pair circulation since the latter""s portion is negative here. A vortex generator can be used, e.g., in order to generate a generator vortex pair with the first spin direction at a landing gear which more or less compensates the portion of a flap vortex pair with the second spin direction in landing flight, so that the circulation of the entire interfering vortex pairs in landing approach is reduced to near the desired value.
One embodiment of the new method arranges at least one winglet pair behind the wings at the fuselage. There the winglets make an impact on those vortex sheets which subsequently are shed from the wing-fuselage junction area.
When at least one winglet pair is arranged at the wing tips of the horizontal tail plane the winglets make an impact on the vortex sheets shed from the horizontal tail plane by enlarging the effective aspect ratio and the effective span of the horizontal tail plane.
When at least one winglet pair is arranged in the inside edge area of a landing flap pair at the wings the winglets make an impact on the shed vortex sheets, which subsequently form the flap vortex pair.
The impact of the winglets on the shed vortex sheets means in any case a reduction of the circulation of those vortex pairs that emerge from the vortex sheets.
As already indicated above in connection with the difference between landing and cruise flight the relative circulation of the interfering vortex pairs is not always the same without intervention by means of winglets according to the present invention. Hence, a fixed arrangement of winglets can only achieve an optimum circulation reduction of the wing vortex pair in certain flight phases. If all three flight phases, i.e. climbing, cruise and landing approach shall be covered, e.g. the active length and/or the angle of attack of at least one winglet pair will have to be modified for an optimal adjustment of the interfering vortex pair circulation to the specific current value. It is also possible to extend individual winglet pairs only under certain flight conditions or to angle them into an active alignment.
The reduction of the relative circulation of the interfering vortex pair according to the present invention must, however, not be made to the extent that the interfering vortex pair circulation is reduced to a value that is only 15% of the wing vortex pair circulation or even smaller. Otherwise the circulation of the interfering vortex pair, even if it does not escape from the altitude range of the descending wing vortices, is too low as to be capable of reliably destroying the wing vortex pair structure.
In a new aircraft also the aspect ratio and the span of the horizontal tail plane can be increased as a means for a premature breakdown of the wing vortex pair. The increase of the aspect ratio and span of the horizontal tail plane as a means for a premature breakdown of the wing vortices has the advantage that the aerodynamic drag is reduced by a reduction of the induced drag.
Also a canard wing can be attached as a means for a premature breakdown of the wing vortex pair. This is an additional horizontal tail plane in front of the wings which provides a lift and not a downward lift like a conventional horizontal tail plane that is located behind the wings. Then the dimensions of the conventional horizontal tail plane and the canard wing are coordinated with each other in such a way that the interfering vortex pair descends with the wing vortex pair.
Alternatively or additionally a short flap can be attached to the wing trailing edge, which ends flush with the fuselage but is free at the other end so that an additional descending vortex pair is generated which increases the effective relative span of the tail plane vortex pair that is shed from the horizontal tail plane.
Below the invention is explained and described in greater detail by means of practical examples.